Duplex stainless steel and cast article thereof

ABSTRACT

A generally Mo-free, ferrite-austenite duplex stainless steel has a ferrite phase area ratio of about 20 to about 60% and a composition containing, in mass %, not more than about 0.08% of C, about 0.5 to about 1.5% of Si, not more than about 1.0% of Mn, about 4.0 to about 8.0% of Ni, about 23 to about 27% of Cr, about 2.0 to about 6.0% of Cu, about 0.05 to about 0.3% of N, and the balance being Fe and generally unavoidable impurities.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims foreign priority benefits under 35 USC §119(a)-(d) from Japanese Patent Application No. 2007-012845, filed onJan. 23, 2007, which is hereby incorporated by reference in itsentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a duplex stainless steelhaving a ferrite phase and an austenite phase and also relates to a castarticle of the duplex stainless steel.

2. Description of the Related Art

A two-phase stainless steel having a ferrite phase and an austenitephase, also referred to as duplex stainless steel, is now used as a rawmaterial of various members because of its excellent corrosionresistance.

JP-B-3270498 proposes a two-phase stainless steel for a large-sizedarticle. The proposed stainless steel is composed of up to 0.02 mass %of C, up to 2.0 mass % of Si, up to 2.0 mass % of Mn, up to 0.04 mass %of P, up to 0.04 mass % of S, 3 to 7 mass % of Ni, 17 to 27 mass % ofCr, 0.5 to 6.0 mass % of Mo, 1 to 5 mass % of Cu, up to 3 mass % of W,0.05 to 0.3 mass % of N, 0.0005 to 0.0015 mass % of B and the balancebeing Fe.

The contents of individual elements of the proposed stainless steel areoptimized to reduce the likelihood of a σ phase, carbides and nitridesforming during the fabrication of a thick cast product, such as apropeller blade for a large marine vessel. The σ phase, carbides andnitrides can form because of a slow cooling rate due to the largethickness of the cast product. The optimization adversely affects bothcorrosion resistance and toughness of the resulting product.

JP-A-H09-302446 proposes a two-phase stainless steel having highmechanical strength and corrosion resistance against seawater. Thedisclosed stainless steel is composed of up to 0.06 mass % of C, 1.5 to3.5 mass % of Si, 0.1 to 3.0 mass % of Mn, 2 to 8 mass % of Ni, 18 to 28mass % of Cr, 0.1 to 0.9 mass % of Mo, 0.03 to 0.2 mass % of N, and thebalance being Fe.

SUMMARY OF THE INVENTION

In the known two-phase stainless steel, the austenite and ferrite phasesare formed by using Cr and various other elements. Namely, the inclusionof various elements within respective ranges required is essential inorder to form the austenite and ferrite phases within a desired ratiothereof.

Of such elements, similar to Cr, Mo is also an essential element toimprove corrosion resistance, especially corrosion against a reducingenvironment, of the two-phase stainless steel. However, Mo is not onlyexpensive but also tends to adversely affect mechanical properties, suchas toughness of the stainless steel, as the content thereof increases.Although an attempt has been made to reduce the Mo content in thestainless steel disclosed in the aforementioned JP-A-H09-302446, Mocontinues to be used in order to obtain the desired corrosionresistance.

Stainless steel has been hitherto used for a cast article having athinner portion thereof, such as a propeller blade of a propulsion unitfor small watercrafts. In producing such a cast article having a thinnerportion, a melt poured into a mold is liable to be cooled at the sectionof the mold containing the thinner portion and the fluidity of the meltis therefore apt to be reduced there. When the difference between thepouring temperature and the melting point of the stainless steel issmall, therefore, the melt cannot flow sufficiently into the thinnerportion forming section. This is likely to cause defects, such asincomplete filling, in the cast product.

If the pouring temperature is increased to ensure a sufficientdifference from the melting point to reduce the likelihood of incompletefilling, then the temperature of the melt introduced into the mold willbe also increased correspondingly, resulting in application of a greatthermal load to the mold and generation of a gas during castingoperation. The use of such a high temperature melt is thereforedisadvantageous.

It is therefore an object of certain features, aspects and advantages ofthe present invention to provide a duplex stainless steel that issubstantially free of Mo, that uses a reduced number of kinds ofelements, and that shows satisfactory mechanical strength and corrosionresistance.

It is another object of certain features, aspects and advantages of thepresent invention to provide a duplex stainless steel capable ofproviding a wide temperature range over which the melt thereof has asuitable fluidity without increasing the pouring temperature and,therefore, capable of easily affording a cast article having a thinnerportion.

To solve the problems mentioned above, an embodiment of a duplexstainless steel can be characterized by having Fe as a major componentand further containing C, Si, Mn, Ni, Cr, Cu, Ni and some generallyunavoidable impurities. The duplex stainless steel also can comprise aferrite phase and an austenite phase with an area ratio of the ferritephase being not less than about 20% but not more than about 60%.

In some configurations, the duplex stainless steel comprises, in mass upto about 0.08% of C, not less than about 0.5% but not more than about1.5% of Si, up to about 1.0% of Mn, not less than about 4.0% but notmore than about 8.0% of Ni, not less than about 23% but not more thanabout 27% of Cr, not less than about 2.0% but not more than about 6.0%of Cu, not less than about 0.05% but not more than about 0.3% of N, andthe balance being Fe and generally unavoidable impurities, andcomprising a ferrite phase and an austenite phase with an area ratio ofthe ferrite phase being not less than about 20% but not more than about60%.

In some configurations, the duplex stainless steel comprises a meltingpoint of not higher than about 1,450° C., and is used as a castingmaterial of a cast article comprising a portion having a thickness ofabout 3 mm or less. Thus, in some configurations, the cast article isformed of duplex stainless steel as described herein. In one embodiment,the cast article can comprise a propeller blade for a propulsion unit ofa small watercraft.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects and advantages of embodiments of thepresent invention will be described below with reference to the attacheddrawings. The drawings comprise two figures.

FIG. 1 is a plan view that schematically illustrates a propeller of asmall watercraft.

FIG. 2 is a phase diagram that schematically illustrates phaseconstitution as a function of the Ni equivalent vs. Cr equivalent.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference now to FIG. 1, a propeller 10 is formed of a certainduplex stainless steel. The propeller comprises a central base section11 and blades 12 that extend in radial, opposing directions from thebase section 11. The blades 12 are formed in one body with the centralbase section 11. Each blade 12 preferably comprises a wide surfaceportion that has a thickness of about 3 mm or less, and more preferablyabout 2 mm or less.

The propeller 10 can be formed using a casting mold that includes twohollow potions, one for forming the central base portion 11 and theother for forming the blades 12. The two hollow portions are positionedsuch that they are in communication with each other. The melt to bepoured into the casting mold includes constituent elements whose amountsare selected to obtain a specified duplex stainless steel of which thepropeller is formed. The propeller 10 is formed by pouring the melt intothe casting mold at a pouring temperature of, for example, about 1,550to about 1,650° C. and then leaving as it is for heat release.

In one preferred embodiment, the duplex stainless steel used in castingthe propeller 10 comprises C, Si, Mn, Ni, Cr, Cu, Ni and the balanceincludes Fe and some generally unavoidable impurities. Thus, thestainless steel is substantially free of expensive Mo. Yet, the duplexstainless steel has a ferrite phase and an austenite phase and a reducedmelting point.

Carbon (C) has been selected because it is highly effective in forming agenerally stable austenite phase. Carbon also improves the strength ofthe duplex stainless steel. If the carbon content is excessively high,however, chromium carbide is apt to be formed, which reduces thecorrosion resistance of the steel and, further, the steel becomesbrittle. Additionally, as the carbon content increases, the meltingpoint of the steel decreases but the strength of the steel is adverselyaffected. Accordingly, the carbon content preferably is low, i.e. up toabout 0.08 mass % in one preferred configuration.

Silicon (Si) is a deoxidizer and is somewhat effective in stabilizingthe ferrite phase. Further, the melting point of the steel decreaseswith an increase of the silicon content. Since Mo, which is aferrite-stabilizing element, is not used in the preferred embodiment,the use of a relatively large amount of silicon is desired. Accordingly,the silicon content is preferred to be high, i.e. not less than about0.5 mass % but not more than about 1.5 mass %.

Manganese (Mn) is a deoxidizer and, as well as nickel, contributes to anincrease of solid solution of nitrogen in the duplex stainless steel.Manganese is also less effective to stabilize the austenite phase. Themelting point of the stainless steel decreases with an increase in themanganese content. The use of manganese in an excessive amount, however,adversely affects corrosion resistance, such as pitching corrosionresistance. Accordingly, the manganese content is preferably up to about1.0 mass %.

Nickel (Ni) improves mechanical properties and moldability, helps tomaintain corrosion resistance and helps to stabilize an austenite phase.Nickel also has a small influence upon the melting point of thestainless steel depending upon the amount of nickel used. Because Mo,which is a ferrite-stabilizing element, preferably is not used in thepreferred embodiment, nickel, which is an austenite stabilizing elementand which has a small influence upon the melting point, preferably isnot used in a large amount. Accordingly, the nickel content preferablyis low, i.e. not less than about 4.0 mass % but not more than about 8.0mass %.

Chromium (Cr) a main component that contributes to impart corrosionresistance to the duplex stainless steel and stabilizes the ferritephase. Higher chromium content results in a better corrosion resistancedue to an improved stability of a passive film. Chromium also has asmall influence upon the melting point of steel depending upon theamount of chromium. Because Mo, which is a ferrite-stabilizing elementand which contributes to corrosion resistance, preferably is not used,chromium, which is a ferrite stabilizing element and which contributesto improved corrosion resistance, preferably is used in a large amount.However, too large an amount of chromium adversely affects themechanical properties and moldability. Accordingly, the chromium contentpreferably is not less than about 23 mass % but not more than about 27mass %.

Copper (Cu) imparts corrosion resistance to the duplex stainless steel.The melting point of the steel can be reduced by increasing the coppercontent. Since Mo, which improves corrosion resistance, preferably isnot used, the copper content preferably is high. However, too high acopper content can cause the steel to become brittle. Accordingly, thecopper content preferably is not less than about 2.0 mass % but not morethan about 6.0 mass %.

Nitrogen (N) can increase the strength of the duplex stainless steeleven in a small amount and is highly effective in stabilizing anaustenite phase. The nitrogen content has little influence upon themelting point of the steel. Too large an amount of nitrogen isundesirable because of the precipitation of nitrides. Accordingly, thenitrogen content is preferably not less than about 0.05 mass % but notmore than about 0.3 mass %.

The balance of the duplex stainless steel is Fe and some generallyunavoidable impurities such as phosphorus (P) and sulfur (S). Suchimpurities may possibly include Mo. As long as the amount of impurity Mois not more than about 0.3 mass %, it may be further removed, or may beused as it is without removal, because the Mo content, which is anunavoidable impurity, is very small.

To obtain the duplex stainless of the present invention, the contents ofthe components described above are selected within the respective rangesthereof to adjust the area ratio of the ferrite phase and the austenitephase. The area ratio of the ferrite phase should be not less than about20% but not more than about 60% in order for the stainless steel toexhibit well balanced corrosion resistance, particularly pittingcorrosion resistance and mechanical strength.

The area ratio of the ferrite phase varies depending upon the coolingrate and other production conditions but the area ratio may be suitablycontrolled by adjustment of mixing proportions of the elements of theduplex stainless steel in terms of the chromium equivalent (ferriteforming elements) and nickel equivalent (austenite forming elements),which may be expressed by the following formulas (1) and (2), forexample:

Cr equivalent=% Cr+% Mo+1.5×% Si+0.5×% Nb  (1)

Ni equivalent=% Ni+30×% C+0.5×% Mn+30×% N  (2)

wherein individual % elements show the contents of these elements interms of mass %, and % Nb (Niobium) is taken into account only when itis present.

FIG. 2 shows a schematic phase diagram that gives the area ratio of theferrite phase as a function of the nickel equivalent compared to thechromium equivalent. In accordance with certain features, aspects andadvantages of the present invention, the proportion of the constituentelements is adjusted so that the chromium equivalent and nickelequivalent fall within the region S where the area ratio of the ferritephase is not less than about 20% but not more than about 60%. Becausethe strength is apt to be reduced due to an excessively small area ratioof the ferrite phase while the corrosion resistance is apt todeteriorate due to excessive large area ratio of the ferrite phase.

Further, the area ratios of the ferrite phase and the austenite phaseare adjusted in such a manner described above and the contents of thecomponent elements also described above are adjusted so that the meltingpoint of the duplex stainless steel of the embodiment is preferably toabout 1,450° C. or lower, more preferably about 1,430° C. or lower. Themelting point preferably is as low as possible. If the melting point isdetermined to be excessively high, then the temperature of the melt mustbe increased. Otherwise the fluidity of the melt would be reduced, whichwould cause difficulties when forming the thin portion of the casting.

The duplex stainless steel described above contains specific content ofC, Si, Mn, Ni, Cr, Cu, N and Fe, and unavoidable impurities. Thedescribed stainless steel also has the ferrite phase and the austenitephase. Moreover, the described stainless steel has an area ratio of theferrite phase in a range between about 20% and about 60%. Therefore, theduplex stainless steel described above has well-balanced mechanicalstrength and corrosion resistance even though Mo is not added thereto.Because the propeller 10 is designed to be brought in contact with wateror seawater, a duplex stainless steel is desired that, while beinginexpensive, is durable in practical use with fewer kinds of componentelements.

The above-described duplex stainless steel advantageously has a meltingpoint of about 1450° C. or less and a wider range of temperature inwhich the melt can flow can be easily secured without increasing thepouring temperature. The thermal load applied to the casting moldtherefore is not necessarily increased and the fluidity of the melt canbe improved. Accordingly, molding defects are less likely to occur inthe blades 12 or other reduced thickness regions even though the blades12 have a portion that is less than about 3 mm thick.

The duplex stainless steel propeller 10 can be fabricated inexpensivelybecause of less elements that make up the stainless steel mixture andthe duplex stainless steel has a good melt fluidity, which reduces thelikelihood of casting defects in the blades 12. Mechanical strength andcorrosion resistance can be secured that are sufficient to resiststresses generated that correspond to a propulsive force in normaltemperature water and seawater.

Stainless steels containing components shown in Table 1 and the balancecontaining Fe and generally unavoidable impurities were prepared andtheir liquid phase line temperatures (as melting points) and area ratiosof the ferrite phase (α phase) and austenite phase (γ phase) wereevaluated by actual measurement and by simulation. The results are shownin Table 1.

TABLE 1 Liquid phase line temperature α γ C Si Mn Ni Cr Cu Mo N (° C.)phase phase Comparative 0.054 0.7 0.58 8.87 22.98 0.15 3.31 0.13 1453 —— Example 1 Comparative 0.05 0.8 0.8 7 25.00 2.5 3.00 0.15 1425 41.8 57Example 2 Example 1 0.05 0.8 0.8 7 25.00 2.5 0 0.15 1425 24.4 74.6Example 2 0.05 0.8 0.8 7 25.00 4 0 0.15 1404 30.2 65.2 Example 3 0.051.2 0.8 6 25.00 4 0 0.15 1409 40.4 54.7 Example 4 0.05 1.2 0.8 6 25.00 40 0.2 1405 34.4 60.4 Example 5 0.05 1.2 0.8 6 25.00 4 0 0.3 1399 28.865.8 Example 6 0.050 1.17 0.82 5.59 25.10 4.00 0 0.15 — 50 50 Note 1: %mass for elementNote 2: measurement value as to Comparative Example 1 and Example 1

Test pieces formed of Example 6 and Comparative Example 1 were testedfor tensile strength and impact resistance in the following manners.

Using the similarly shaped test pieces, a tensile test was carried outusing the metallic material tensile test in accordance with JIS Z2371.In addition, using similarly shaped test pieces, an impact test wascarried out by metallic material impact test in accordance with JISZ2371. The test results are summarized in Table 2.

TABLE 2 Tensile Elongation strength 0.2% Proof after fracture Impactstrength (MPa) strength (MPa) (%) (J/cm²) Comparative 654 430 12.7 66.4Example 1 Example 6 734 450 11.3 105

As is evident from the results shown in Table 2, the generally Mo-freeduplex stainless steel of Example 6 has an equal or greater tensilestrength and impact strength as compared with the Mo-containingstainless steel of Comparative Example 1.

Using the stainless steels of Example 6 and Comparative Example 1,propellers as shown in FIG. 1 were prepared by casting. The minimumthickness of the blades 12 of the propeller 10 was 1.6 mm. As a result,a good cast propeller could be obtained using the stainless steel ofExample 6. On the other hand, due to a high melting point, the blades 12made of the stainless steel of Comparative Example 1 had casting defectswhen the same pouring temperature was used to thereby reduce thetemperature range. Thus, it was revealed that the stainless steel ofExample 6 was able to give a thin cast article more easily than that ofComparative Example 1.

The propellers 10 made of the stainless steels of Example 6 andComparative Example 1 were each subjected to a corrosion test. Anaqueous brine solution spray test as the corrosion test was carried outunder conditions in accordance with JIS Z 2371. Thus, a 5% by weightaqueous brine solution having a temperature of 35° C. was sprayed overthe test piece. The test piece was then allowed to stand for 4 days tocheck rust formation with unaided eyes. No rust was observed on surfacesof the test pieces of Example 6 and Comparative Example 1. Thus, it wasrevealed that the stainless steel of Example 6 has corrosion resistancesimilar to the stainless steel Comparative Example 1.

Although the present invention has been described in terms of certainembodiments, other embodiments apparent to those of ordinary skill inthe art also are within the scope of this invention. Thus, variouschanges and modifications may be made without departing from the spiritand scope of the invention. For instance, various components may berepositioned as desired. Moreover, not all of the features, aspects andadvantages are necessarily required to practice the present invention.Accordingly, the scope of the present invention is intended to bedefined only by the claims that follow.

1. A duplex stainless steel characterized by comprising Fe as a majorcomponent and further containing C, Si, Mn, Ni, Cr, Cu, Ni and generallyunavoidable impurities, and comprising a ferrite phase and an austenitephase with an area ratio of the ferrite phase being not less than about20% but not more than about 60%.
 2. A cast article of a duplex stainlesssteel comprising the duplex stainless steel according to claim
 1. 3. Acast article in the form of a propeller blade for a propulsion unit of asmall watercraft, the cast article comprising the duplex stainless steelaccording to claim
 2. 4. A duplex stainless steel according to claim 1,characterized by having a melting point of not higher than about 1,450°C., and characterized by being used as a casting material of a castarticle comprising a portion having a thickness of about 3 mm or less.5. A cast article of a duplex stainless steel comprising the duplexstainless steel according to claim
 4. 6. A cast article in the form of apropeller blade for a propulsion unit of a small watercraft, the castarticle comprising the duplex stainless steel according to claim
 5. 7. Aduplex stainless steel characterized by comprising, in mass %: up toabout 0.08% of C, not less than about 0.5% but not more than about 1.5%of Si, up to about 1.0% of Mn, not less than about 4.0% but not morethan about 8.0% of Ni, not less than about 23% but not more than about27% of Cr, not less than about 2.0% but not more than about 6.0% of Cu,not less than about 0.05% but not more than about 0.3% of N, and thebalance being Fe and generally unavoidable impurities, and characterizedby comprising a ferrite phase and an austenite phase with an area ratioof the ferrite phase being not less than about 20% but not more thanabout 60%.
 8. A cast article of a duplex stainless steel comprising theduplex stainless steel according to claim
 7. 9. A cast article in theform of a propeller blade for a propulsion unit of a small watercraft,the cast article comprising the duplex stainless steel according toclaim
 8. 10. A duplex stainless steel according to claim 7,characterized by having a melting point of not higher than about 1,450°C., and characterized by being used as a casting material of a castarticle comprising a portion having a thickness of about 3 mm or less.11. A cast article of a duplex stainless steel comprising the duplexstainless steel according to claim
 10. 12. A cast article in the form ofa propeller blade for a propulsion unit of a small watercraft, the castarticle comprising the duplex stainless steel according to claim 11.